In some animals males harm their mates during mating, either physically or with toxic seminal fluids. Harming the mother of your future offspring seems like a bad idea, but it may benefit males if injured females refrain from mating again with a different male (because they can’t risk additional injuries) or if females perceive the injuries as a threat to their survival and try and rapidly produce as many offspring as possible before they die. In both cases, males benefit through the increased number of eggs fertilized by their sperm before the females remate and their sperm have to compete with sperm from other males.
Males of the bruchid beetle Callosobruchus maculatus have spines on their genitalia that puncture the female reproductive tract during mating. Females kick their mates during copulation and sustain more extensive injuries if kicking is prevented. We found that these injuries are costly to females since they produced fewer offspring over their lifetimes when they were prevented from kicking. However, we could find no evidence that males benefited from harming their mates. Females that were prevented from kicking did not respond to the extra harm by delaying remating or increasing their rate of egg laying.
Our study suggests that the injuries inflicted by males are simply side effects of another function of the spines such as serving as an anchor during copulation. To males, avoiding being dislodged by other males may outweigh the costs of reducing the offspring production of their mates by harming them.